Jigs

Jigging is one of the oldest methods of gravity concentration. The elementary equipment is a hindered settling device consisting of shallow, flat trays with perforated bottoms that contains layers of high density material such as particles of steel balls through which water pulsates up and down. The direction of flow through the bed is reversed several times. In this way, the bed is dilated by the forward stroke of a plunger and compacted by the plunger’s backward stroke.

In general, this equipment uses the differential acceleration of particles of different specific gravity to separate gold minerals and gangue. The slurry containing the minerals to be concentrated is fed onto a permeable screen. The jig operates by the periodic movement of water pulse through the screen. Stratification in a bed of particles results from the repeated pulsation of a current of fluid up through the bed. The particles in the bed are expanded so that when pulsation finish, the particles are allowable to consolidate under the influence of gravity. The expansion and contraction is repeated in a cycle operation until the heavy and light particles are stratified according to their densities (specific gravity).

The jigs operate by the periodic movement of water through the screen. The upward velocity of the water brings all the particles into a suspension with particles surrounded by liquid. The water maintained in this movement and then allowed to drain back through the grid. The bed of particles collapses back onto the screen and differential acceleration of particles occurs during this stage of the process. The next figure shows the expansion and contraction of bed particles.

Typically jigs are able to treat coarse material and for this reason they are located in the grinding circuit. They can receive slurry from the ball mill discharge or hydrocyclone underflow. The idea is to recover gold minerals soon in order to avoid overgrinding. The concentrate is collected at the bottom of the jig and the tails are moving out of the jig at the top of the bed. Although most gold is collected at the bottom, coarse gold particles like nuggets can stay at the top due to the screening opening size is smaller than the nugget.

Diagram of a jig

The conventional jig is a high capacity concentrator that can separate material from one inch down to about 150 µm, although significant recovery of gold finer than 80 µm has been reported. Jigs can process 7-25 t/h depending on their size with recoveries around 70%. A common configuration is a double line of four cells in series, and each two cells driven by an eccentric box provided with a geared motor. The design of conventional jigs differs mainly in the placement of the plunger and the jig bed and in where the makeup water enters the jig. In this way, a design can include a plunger with a short compression stroke and a long, slow suction stroke. This configuration modifies the jigging process.

Jigs can receive coarse particles such as a ball mill discharge. The jig concentrate is sent to the concentrate thickener or leaching depending on the gold ore type. Jig tails are classified and reground. Next figure shows a comminution circuit including a jig.

Grinding circuit with jigs